With a recent rapid development of a display technology of a liquid crystal display device or the like, demand for one having a novel function or higher quality is also becoming strong for a sheet-like optical member used therein. Examples of such sheet-like optical member include a prism sheet used for a back light of a liquid crystal display device or the like, a lenticular lens sheet used for stereograph, a projection screen or the like, and a Fresnel lens sheet used for a condenser lens of an overhead projector or the like. Such a sheet-like optical member usually has a substrate; and a lens part that is formed on the substrate and has unit prisms formed with a resin having the predetermined refractive index into a fine convexo-concave shape on a surface thereof.
The sheet-like optical member refracts light in the lens part to develop a predetermined function, and, in accordance with its application, a resin material constituting the lens part and a shape of the unit prism are determined. Accordingly, a shape of the unit prism is important from the viewpoint of developing a function of a sheet-like optical member. In the course of producing the sheet-like optical member, it has been necessary to give close attention so as to avoid collapse in a shape of the lens part or wear caused by friction. In this connection, as a material used in the lens part, a material that has high elastic modulus and is difficult to deform a shape of a formed unit prism has been used.
As a material used in the lens part, for example, urethane acrylates, each of which has a predetermined structure, are described in Patent Documents 1 to 3. Further, in Patent Document 4, unsaturated group-containing compounds having an S atom and two or more unsaturated groups, wherein a content of S atoms and a glass transition temperature in predetermined ranges are described. As the materials develop the elastic modulus to a certain extent by curing, they contribute to inhibit a shape of the lens part from being damaged.
However, with the foregoing rapid advancement of the display device, further improvements in the productivity and the like are demanded also for the sheet-like optical member. In this connection, a lens part formed from the foregoing material is still insufficient in the elastic modulus. Accordingly, when such a material is used, in order to inhibit the unit prism from deforming in the course of preparing a sheet-like optical member, a process consideration has to be paid so that excessive pressure may not be applied on the lens part or the lens part may not glaze with other member to be worn out.
From such restrictions in the process, the productivity of the sheet-like optical member has not been necessarily high.
Further, in recent years, display devices such as liquid crystal TVs, or electric advertisement panels are in a tendency to be larger in area. Therefore, research developments and material developments of accompanying associated parts have been actively forwarded. Such a display device has a surface light source device irradiating light from a rear side, and the surface light source device includes an optical sheet that refracts and transmits light from a light source toward a display panel. Also in the optical sheet, according to a tendency toward enlarging the area, for example, a wide and long optical sheet is produced and wound in a roll.
The optical sheet has a lens part formed by arranging a plurality of unit prisms, and is disposed on a light emitting surface of a display panel side in both of an edge light type surface light source device and a direct type surface light source device. An edge light type surface light source device is a device constituted such that source light is input from one side edge surface of a planar light guide body usually made of a transparent acryl resin or the like and light is output from an outputting surface that is one surface of the light guide body toward a back surface of a liquid crystal panel or the like. On the other hand, a direct type surface light source is a device constituted such that a liquid crystal panel and a reflective plate are disposed with a light source interposed therebetween, and, usually, light from a light source is reflected by a reflective plate toward a back surface of a liquid crystal panel or the like.
An optical sheet is produced as a wide and long sheet, and wound in a roll at the production. At that time, owing to its own weight of the roll, particularly at a winding core part, there is a problem that a pinnacle of a unit prism is “collapsed”.
Furthermore, there is also a problem that, when a lens part provided to an optical sheet and a light guide body provided to surface light source device come into contact, a pinnacle of a unit prism is collapsed by heat added in the process. Moreover, there is another problem that a lens part and a light guide body come into contact to cause a crack of a pinnacle of a unit prism.
Problems of deformation or crack of a pinnacle of a unit prism generate display irregularities such as white spots (white pattern) on a display surface of a display device to deteriorate display performance. Therefore, a countermeasure thereof is in demand.    Patent Document 1: Japanese Patent Application Publication Laid-Open (JP-A) No. 2002-105149    Patent Document 2: JP-A No. 2004-131520    Patent Document 3: JP-A No. 2005-263913    Patent Document 4: JP-A No. 2006-72346    Patent Document 5: JP-A (Japanese Translation of PCT International Application) No. 11-500072